1. Field of the Invention
The present invention relates to a magnetic recording medium for use in hard disk drives or the like. More particularly, it relates to a magnetic recording disk having a fine surface irregularity structure (texture) on its surface, and a method of manufacturing such a magnetic recording disk.
2. Description of the Related Art
Magnetic recording disks for use in conventional hard disk drives or the like comprise an Ni--P layer, a magnetic layer, a protective film, a surface lubricating layer, etc. successively formed on a surface of a substrate made of an aluminum material or the like. In recent years, a glass substrate has been finding practical use in place of the aluminum substrate to meet demands for hardness and smoothness.
The above hard disk drives have heads with a minimized lift, i.e., a semicontact head, in view of ever-increasing demands for higher recording density. Therefore, it has been desired to make the surface of magnetic recording disks smoother. However, if the surface of magnetic recording disks is too smooth, it attracts the head to the extent the head sticks inseparably to the surface. As a result, the surface of magnetic recording disks should preferably have certain microscopic surface irregularities (texture).
The texture of conventional magnetic recording disks has been formed by a mechanical abrading process to abrade the surface of the Ni--P layer or the like on the aluminum substrate with an abrasive tape or an abrasive cloth.
If an abrasive tape is used, then the abrasive tape carries abrasive grain bonded to its opposite surfaces to abrade the surface of the disk. Specifically, the abrasive tape is pressed against the surface of the disk, and the surface of the disk is rotated to produce abrasive grooves thereon for thereby forming a texture structure composed of fine surface irregularities. If an abrasive cloth is used, then an abrasive solution containing abrasive grain is dropped onto the abrasive cloth, and the surface of the disk to be abraded is pressed against the abrasive cloth and rotated to produce abrasive grooves thereon.
Fine surface irregularities (texture) are formed on the surface of a magnetic recording disk having a glass substrate by dissolving the surface with a vapor of hydrofluoric acid or crystalline deposition. When a texture is formed on the surface of a magnetic recording disk by dissolving the surface with a vapor of hydrofluoric acid, the surface is dissolved at locally different rates due to different vapor adsorption or density rates to form fine surface irregularities on the surface. When crystalline deposition method is employed, the size of crystal grain in the glass substrate is controlled by heat treatment to separate crystal grain irregularities on the surface, turning the surface into a fine surface irregularity structure.
In the mechanical texture forming processes using the abrasive tape or the abrasive cloth, the accuracy is poor, and the surface irregularities suffer large variations. These problems make it difficult to use a semicontact head whose lift is about 30 nm.
It has been attempted to apply a laser beam to form the fine surface irregularity structure (texture). Specifically, a laser beam having a very small diameter focused by a lens and a focusing mirror is scanned over and applied to the surface of the disk to concentrate the energy of the laser beam on a small area. The concentrated laser beam brings about a laser abrasion phenomenon to produce crater-like surface irregularities at regular intervals as the texture structure.
Usually, a pulse laser is used to produce the above laser abrasion phenomenon. Height variations of the surface irregularities may possibly be caused by variations of laser spot diameter and laser beam intensity depending on the position on the disk, and time-dependent variations of laser intensity. A limitation on the number of times that the laser beam can repeatedly be applied (the frequency at which the laser beam is applied) is considered to result in a limitation on the number of surface irregularities. Therefore, it needs a long process time to form a texture structure on the entire disk surface with a laser beam. When a surface irregularity structure having a height of 20 nm or less is needed as a fine texture for a high magnetic recording density, the frequency at which the pulsed laser beam is to be repeatedly applied usually ranges from 1 to 10 kHz. Consequently, if surface irregularities are to be formed in the shape of regular triangles at a pitch of 10 .mu.m, then about 115 tens of thousands of surface irregularities have to be produced per 1 cm.sup.2. It takes about 1156 second/cm.sup.2 if the repetition frequency is 1 kHz, and 115 second/cm.sup.2 if the repetition frequency is 10 kHz. Thus, it requires a long process time to form a texture structure on the entire disk surface, resulting in problems of high cost and process time, which make the process not practical.
When a metal surface is left to stand in the atmosphere, a natural oxide layer is usually formed on the outermost surface layer. Though the natural oxide layer takes a different form depending on the material thereof, a natural oxide layer having a thickness of 3-5 nm is formed on an Ni--P layer on an aluminum material. In the process based on the laser abrasion phenomenon, the natural oxide layer and a non-oxide layer have different characteristics to absorb and reflect the laser beam. Therefore, when the disk surface is melted by the laser abrasion, the natural oxide layer and the non-oxide layer have different melting properties. Variations of the depth of focus of the laser beam greatly affect variations of the height of produced fine surface irregularities, making it difficult to form fine surface irregularities that are of a uniform height.
Efforts have been made to form the texture with an energy ion beam. According to this process, an ion beam or plasma is used to produce a fine surface irregularity structure, and generated ions are electrically accelerated and applied to the disk surface for thereby processing the disk surface. A resist film pattern according to the photolithography technology is used to pattern the disk surface to form the fine surface irregularity structure. Since, however, the resist film pattern is generally of an insulating material, it suffers the problem of being charged up.
As described above, a natural oxide layer is formed on the outermost layer of a metal surface. Because the natural oxide layer is highly insulative, it tends to be charged up in the process which uses an ion beam or plasma, causing changes in the path of energy ions and fluctuations of the characteristics of the applied energy. As a consequence, the surface roughness is impaired, and the amount of local processing is not made uniform.
There are also processes of forming a surface irregularity structure on the surface of a glass substrate by dissolving the surface with a vapor of hydrofluoric acid or a method of crystalline deposition. These processes are also disadvantageous in that the accuracy is poor and the surface irregularities suffer large variations, which also make it difficult to use a semicontact head.
Specifically, because the process to dissolve the surface with a vapor of hydrofluoric acid uses variations of the amount of the vapor adsorbed to the glass substrate, the vapor density nonuniformities cause variations of the depth to which the surface is dissolved, increasing processed surface depth nonuniformities. Consequently, it is impossible to form fine surface irregularities having a height of 20 .mu.m, for example, over the entire surface of the glass substrate with a uniform processed surface depth and a uniform density. The process based on crystalline deposition controls the crystal form of the glass through heat treatment for controlling the size and quantity of crystal particles separated on the surface according to the temperature. It is difficult to form fine surface irregularities over the surface of the glass substrate with a uniform processed surface depth and a uniform density because of nonuniform heat treatment, nonuniform crystal grain size, and nonuniform separated particles.